In various optical applications, signals carried by beams having two separate wavelengths are transmitted along parallel directions and received via a common input aperture. These signals must then be processed in two different channels. A first channel is designed to receive the beam associated with one of the wavelengths, and some of the energy of the beam associated with the other wavelength. The second channel is designed to receive the complementary part of the energy of the beam associated with said other wavelength. In other words, the beam having the first wavelength is only intended for the first channel, whereas the beam having the second wavelength is intended for both channels. Furthermore, it is often necessary for the distribution of the energy of the beam having the second wavelength to be able to be adjusted between the two channels.
The free-field optical communication is an example of an application that requires such a distribution of two beams corresponding to different wavelengths. This may for example be optical communication between two satellites, or else between a satellite and a ground or airborne station. As is known, the principle phases of such communication are acquisition, tracking and communication. The acquisition phase consists in directing two optical communication terminals each toward the other. This phase is based on receiving, via each terminal, beacon signals emitted by the other terminal at a first wavelength. Each terminal then receives optical signals emitted by the other terminal at a second wavelength. These signals transmitted at the second wavelength make it possible, on the one hand, to refine the pointing direction of the terminal and, on the other hand, to receive useful data. The refinement of the pointing direction constitutes the tracking phase, and reception of the data corresponds to the communication phase. The tracking and communication phases are generally carried out simultaneously.
Within each terminal, the acquisition and tracking phases are carried out by processing, via a first channel, all the optical signals received, which have the first or the second wavelength. The communication phase is carried out by processing the optical signals received at the second wavelength via a second channel, separate from the first channel.
Hitherto, the fraction of the energy of the signals associated with the second wavelength, which is directed toward the second channel, in other words toward the communication channel, is determined by a semireflective plate placed at the input of the terminal. The distribution of the energy of the signals associated with the second wavelength between the two channels is therefore set during construction of the terminal, and can no longer be adjusted during use of the latter.
Now, if the signals received are of low power or if the terminal is not accurately pointed toward the other terminal, for example when the latter is moving rapidly, it is advantageous to be able to increase the fraction of the energy of the signals associated with the second wavelength which is directed toward the communication channel. Such an adjustment may also be necessary when the transmission conditions between the two terminals vary or are unfavorable.
It is an object of the present invention therefore to provide an optical device for directing signals received at two wavelengths toward two separate channels, with adjustable energy distribution in the case of one of the wavelengths.